Injection molding is the process in which polymeric material is melted and, by means of a rotating screw, injected into a mold cavity that is created by a mold that generally comprises upper and lower mold parts. The material cools and solidifies into a shape conforming to the cavity. The cast or injection molded part, which is then ejected from the molding tool, is usually finished and requires no further steps before being used in an assembly or as an end product.
However, a persistent injection molding problem is sink marks, common to thick parts or parts with varying thick and thin sections. Consequently, as far back as 20 years ago, injection molding processes were conceived in which gas is injected into the plastic melt within the mold cavity to counteract sink marks by creating hollow cores, thus making the part sections more alike in thickness as well as reducing part weight.
Interest in gas injection has revived in recent years due to the development of processes in which continuous pressure of the gas is the control factor for the gas injection which results in parts that come out of the injection mold free of blemishes.
In these gas-assisted injection molding processes, nitrogen, an inexpensive inert gas, is commonly introduced into the plastic melt in the mold cavity, and in known processes, the gas is introduced into the plastic melt through a gas injection nozzle. The gas does not mix with the plastic melt, but takes the line of least resistance through the less viscous parts of the plastic melt. The plastic melt is pushed against the walls of the mold cavity, leaving hollow cores or channels within the part.
With such gas assist techniques, hollow parts and parts with heavy ribs and bosses can be achieved with low in-mold stresses, reduced part warpage, and the elimination of sinks. Gas-assisted injection molding also offers potential for material savings, lower clamp tonnage requirements and reduced cooling/cycle times.
However, if the plastic melt does not seal around the tip of the gas injection nozzle, the path of least resistance may lead to a surface of the mold cavity, in which case the gas injected into the mold cavity through the gas injection nozzle flows through the plastic melt and directly against the surface of the mold cavity. The gas may then force the plastic melt up and away from the mold surface, resulting in a part that does not have the proper gas penetration and/or a part that is deformed or warped at the gas injection nozzle and/or a part that is not fully packed out.